95 suburban howl at 45-50 mph

wondering if any one has any insight , my 95 suburban with 188000 miles seem to run ok the transmission is the greatest but it's still ok . i get about 16 mpg driving conservatively . but when i am cruising a bout 40 ,45-50 , not accelerating there seems to be a howling noise . it isn't alarmingly loud , i am probably the only one of my family that notices it, when i hear it if give a little throttle it goes away and if i let off it comes back . also it only does after having drove 8-12 miles at high speed or close . it sounds to me as if may be coming from the direction of the transfer case , but i am still not overly sure .